1. Field of the Invention
This invention relates in general to spin valve magnetoresistive sensors for reading information signals from a magnetic medium and, in particular, to a dual/differential spin valve sensor with a single AFM layer.
2. Description of the Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR sensors, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an xe2x80x9cMR elementxe2x80x9d) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., Nixe2x80x94Fe) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect.
FIG. 1 shows a prior art SV sensor 100 comprising end regions 104 and 106 separated by a central region 102. A first ferromagnetic layer, referred to as a pinned layer 120, has its magnetization typically fixed (pinned) by exchange coupling with an antiferromagnetic (AFM) layer 125. The magnetization of a second ferromagnetic layer, referred to as a free layer 110, is not fixed and is free to rotate in response to the magnetic field from the recorded magnetic medium (the signal field). The free layer 110 is separated from the pinned layer 120 by a nonmagnetic, electrically conducting spacer layer 115. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed on hard bias layers 130 and 135, respectively, provide electrical connections for sensing the resistance of SV sensor 100. IBM""s U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a GMR sensor operating on the basis of the SV effect.
Another type of spin valve sensor is an antiparallel (AP) spin valve sensor. The AP-pinned valve sensor differs from the simple simple spin valve sensor in that an AP-pinned structure has multiple thin film layers instead of a single pinned layer. The AP-pinned structure has an antiparallel coupling (APC) layer sandwiched between first and second ferromagnetic pinned layers. The first pinned layer has its magnetization oriented in a first direction by exchange coupling to the antiferromagnetic pinning layer. The second pinned layer is immediately adjacent to the free layer and is antiparallel exchange coupled to the first pinned layer because of the minimal thickness (in the order of 8 xc3x85) of the APC layer between the first and second pinned layers. Accordingly, the magnetization of the second pinned layer is oriented in a second direction that is antiparallel to the direction of the magnetization of the first pinned layer.
The AP-pinned structure is preferred over the single pinned layer because the magnetizations of the first and second pinned layers of the AP-inned structure subtractively combine to provide a net magnetization that is less than the magnetization of the single pinned layer. The direction of the net magnetization is determined by the thicker of the first and second pinned layers. A reduced net magnetization equates to a reduced demagnetization field from the AP-pinned structure. Since the antiferromagnetic exchange coupling is inversely proportional to the net pinning magnetization, this increases exchange coupling between the first pinned layer and the antiferromagnetic pinning layer. The AP-pinned spin valve sensor is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin which is incorporated by reference herein.
There is a continuing need to increase the MR coefficient and reduce the thickness of GMR sensors. An increase in the spin valve effect and reduced sensor geometry equates to higher bit density (bits/square inch of the rotating magnetic disk) read by the read head.
It is an object of the present invention to disclose a dual/differential spin valve sensor having a single AFM layer providing pinning of both a first AP-pinned layer structure and a second simple pinned layer of first and second spin valve structures, respectively.
It is another object of the present invention to disclose a dual/differential spin valve sensor with first and second spin valve structures having oppositely oriented pinned layer magnetization directions.
It is yet another object of the present invention to disclose a dual/differential spin valve sensor with first and second spin valve structures having first and second free layers separated by a distance equal to half the bit length of magnetic data recorded on a magnetic recording media.
It is a further object of the present invention to disclose a dual/differential spin valve sensor having first and second free layers biased to provide 90xc2x0 relative orientation difference of their magnetizations at the quiescent bias point (i.e. with no signal field present).
In accordance with the principles of the present invention, there is disclosed a dual/differential spin valve sensor comprising a first spin valve structure, a second spin valve stucture and a single antiferromagnetic (AFM) layer disposed between the first and second spin valve structures. The first spin valve structure comprises a first ferromagnetic layer (FM1), an AP-pinned layer structure having second and third ferromagnetic layers (FM2 and FM3) separated by an antiparallel coupling (APC) layer, and a conductive first spacer layer disposed between the first and second ferromagnetic layers. The second spin valve structure comprises fourth and fifth ferromagnetic layers (FM4 and FM5) separated by a conductive second spacer layer. The AFM layer is sandwiched between the third and fourth ferromagnetic layers and is exchange coupled to the third and fourth ferromagnetic layers providing an exchange field to pin the magnetization directions of the third and fourth ferromagnetic layers in one direction. Due to the antiferromagetic coupling of the APC layer, the magnetization direction of the second ferromagnetic layer is oriented antiparallel to the magnetization direction of the third ferromagnetic layer. Having an AP-pinned layer for the first spin valve structure and a simple pinned layer for the second spin valve structure leads to a 180xc2x0 phase difference between the pinned second and fourth ferromagnetic layers. The first and fifth ferromagnetic layers are free to rotate in response to signal magnetic fields from magnetic data recorded on magnetic media.
The bit transition length of magnetic data recorded on the magnetic media is arranged to be equal to the spacing between the first and fifth ferromagnetic layers (the free layers) of the dual/differential sensor. With the bit transition length equal to the spacing between the first and fifth ferromagnetic layers, the signals generated by the first and second spin valves add due to the 180xc2x0 phase difference of the pinning of the second and fourth ferromagnetic layers. The responses of the first and second spin valves are additive for both longitudinal and perpendicular recording applications as long as the bit transition length is made equal to the separation of the first and fifth ferromagnetic layers.
For optimal sensor performance, the ferromagnetic free layers, FM1 and FM5, should be biased to provide 90xc2x0 relative orientation between the magnetizations of FM1 and and the pinned layer FM2, and similarly, between the magnetizations of FM5 and the pinned layer FM4 at the quiescent bias point (i.e. without any bit field present). This condition can be realized with the dual/differential spin valve sensor of the present invention because the ferromagnetic coupling fields (HF) from the pinned layers FM2 and FM4 and the fields from the sense current (HI) oppose each other at both free layers (FM1 and FM5) for the proper choice of sense current polarity. Demagnetization fields (HD) from the pinned layers FM2, FM3 and FM4 are made to cancel at the free layers FM1 and FM5 by selecting the thicknesses of the pinned layers so that the net demagnetization field HD3-HD2 from the AP-pinned layers FM2 and FM3 cancels the demagnetization field HD4 of the pinned layer FM4 at the free layers.
An advantage of a dual/differential spin valve sensor having a single AFM layer providing pinning for an AP-pinned layer of a first spin valve structure and for a simple pinned layer of a second spin valve structure is that setting a single AFM layer is simpler to fabricate than setting two different AFM materials to generate 180xc2x0 out of phase pinned layers. In addition, the use of a single AFM layer for pinning both spin valve structures results in a significantly thinner dual/differential sensor which translates to a higher bit density read capability for the sensor.
Because of the differential operation of this sensor, stray magnetic fields do not generate any signal. Therefore, there is no need for ferromagnetic shields on either side of the dual/differential sensor of the present invention.
The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed description.